How to Become Fit?
   Healthy eating habit along with regular exercise is the only way to become fit. If you have the correct number of calories and exercise regularly, you will lose weight, keep fit, and feel great, too.
   During your teenage years, it is important to give your body the energy it needs. If you skip (跳过) meals, you won’t get enough calories, and then you will feel tired. By the way, calorie is just another word for energy. About 50% of your calories should come from rice, bread, vegetables and fruit. You also need to drink a lot of water, 6 to 8 glasses a day. Water helps keep your system clean. Drinking enough water will make your skin better and make you healthy.
   Exercise is something that can help to make you look good, feel good and be healthy. Experts suggest that teenagers spend at least 30 minutes exercising, five times a week. That is easy to do! You can walk or ride your bike to school, and do sports at school. However, teenagers often give up sports, saying they have no time left after their studies. In fact, doing exercise can help them sleep better at night and increase their ability to focus on their study.
   If you follow the suggestions above, you will look and feel much better and become fit.

有如下四个命题：

①甲乙两组数据分别为甲：28，31，39，42，45，55，57，58，66；乙：29，34，35，48，42，46，55，53，55，67，则甲乙的中位数分别为45和44．

②相关系数r=﹣0.83，表明两个变量的相关性较弱．

③若由一个2×2列联表中的数据计算得K²的观测值k≈4.103，那么有95%的把握认为两个变量有关．

④用最小二乘法求出一组数据（x_i，y_i），（i=1，…，n）的回归直线方程$\stackrel{\wedge }{y}$=$\stackrel{\wedge }{b}$x+$\stackrel{\wedge }{a}$后要进行残差分析，相应于数据（x_i，y_i），（i=1，…，n）的残差是指$\stackrel{\wedge }{e_i}$=y_i﹣（$\stackrel{\wedge }{b}$x_i+$\stackrel{\wedge }{a}$）．

以上命题“错误”的序号是____ ．